PDFViewer: Add a toolbar

The toolbar has an option to toggle the sidebar, a number input to
set the current page, and two buttons to go up and down by one page

Userland/Applications/PDFViewer/NumericInput.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2021, Matthew Olsson <mattco@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NumericLimits.h>
+#include <LibGUI/TextBox.h>
+
+class NumericInput final : public GUI::TextBox {
+    C_OBJECT(NumericInput)
+public:
+    NumericInput();
+    virtual ~NumericInput() override = default;
+
+    Function<void(i32)> on_number_changed;
+
+    void set_min_number(i32 number);
+    void set_max_number(i32 number);
+    void set_current_number(i32 number, bool call_change_handler = true);
+
+private:
+    void on_focus_lost();
+
+    bool m_needs_text_reset { false };
+    i32 m_current_number { 0 };
+    i32 m_min_number { NumericLimits<i32>::min() };
+    i32 m_max_number { NumericLimits<i32>::max() };
+};